Adam Lanza, the shooter who massacred 20 children and six adults last Friday at Sandy Hook Elementary School, spent hours in the windowless basement of his home playing violent video games like “Call of Duty” and obsessing over “guns and military equipment.”

Peter Wlasuk, a plumber who worked at the Lanza home, toldThe Sun that Lanza “lived in the basement” and he found it “strange.”

“He had a proper set up down there — computers, a bathroom, bed and desk and a TV,” Wlasuk said. “There were no windows.”

Wlasuk said he went down to the basement on numerous occasions while doing work at Lanza’s home and said Lanza also had a “huge poster with every tank ever made” and another one with “every piece of military equipment the U.S. ever made.” He said the Lanza brothers could tell “you about guns they had never seen from the 40s, 50s and 60s.”

Lanza was suspected of being mentally ill and may have had Asperger’s. His high school assigned him to a counselor because they feared he may harm himself but not others.

According to reports, Lanza’s mother was a “a gun enthusiast who owned at least five weapons” and “took her younger son to a local firing range just days before the rampage.”

A family friend told NBC that Lanza’s mother said she wanted to introduce Adam to guns to teach him a “sense of responsibility.”

“Guns require a lot of respect and she really tried to instill that,” the friend said. “And he took to it. He loved being careful with them. He made it a source of pride.”